This invention relates generally to hydraulic systems and, more particularly, to systems for charging and re-circulating hydraulic fluid between hydraulic systems and hydraulic fluid reservoirs.
Today""s earthmoving and agricultural machine hydraulic systems generally use a non-pressurized tank as a reservoir for the hydraulic working fluid to be supplied to a drive pump. For hydrostatic drive hydraulic systems, a charge pump typically is required to charge the drive pump inlet at generally in the 0.7-2.1 MPa (xcx9c100-300 psi) range. This prevents pump cavitation, but also results in power lost due to having to throttle this flow back to the non-pressurized tank across a relief valve. Typically, the charge pump flow represents about 15% of the rated flow of the hydrostatic drive pump.
In the case of implement hydraulic systems, the implement pump is generally designed such that it does not require that its inlet be charged. However, pump rotation speed often must be limited to prevent inlet cavitation. This also puts limitations on tank placement in relation to the pump suction inlet.
Cylinder voiding is another problem frequently encountered using atmospheric drain pressure in conventional implement hydraulic systems. While makeup check valves can be used, large makeup flows are difficult to accomplish with only atmospheric pressure. Installing a charge pump for an implement system generally is not practical, since it would require a large pump (hence more power loss) to effectively deal with the large flows associated with activation/deactivation of implements with large cylinder capacity, such as booms, etc. However, one oft-used solution is the installation of a spring-loaded check valve in the drain line in an attempt to control the drain or recirculation of hydraulic fluid back to the reservoir/tank. Not only does this conventional solution waste power, but it is not effective in all circumstances.
Moreover, most machines having hydrostatic drives have used separate pumps and other fluid control components for the implement and hydrostatic drive hydraulic systems. This is because of the differing requirements of the implement and hydrostatic drive systems respectively. For example, hydrostatic drive hydraulic systems typically require xe2x80x9cover-centerxe2x80x9d pump operation and a xe2x80x9cmotorablexe2x80x9d pump capability, while implement hydraulic systems do not. However, while hydrostatic systems typically need not accommodate large working fluid volume changes, implement systems routinely encounter such changes, as mentioned previously.
The present invention is directed to apparatus and methods that can optionally diminish one or more of the problems or disadvantages associated with the prior art.
In one aspect of the present invention, an apparatus is provided for charging a hydraulic system from a fluid reservoir. The apparatus includes a pump having an inlet and an outlet, a first conduit fluidly connected to the pump inlet and configured to be fluidly connected to the fluid reservoir, and a second conduit fluidly connected to the pump outlet and configured to be fluidly connected to the system. The apparatus also includes an accumulator operatively connected to the second conduit, a third conduit interconnecting the first conduit and the second conduit, and an electrically actuated fill valve operatively disposed in said third conduit.
In another aspect of the present invention, an apparatus is provided for charging and recirculating fluid between a hydraulic system and a reservoir. The apparatus includes a supply conduit having a system end connectable to the system and a reservoir end connectable to the reservoir, a pump operatively disposed in the supply conduit between the system and reservoir ends, a check valve operatively disposed in the supply conduit between the pump and the reservoir end to prohibit return flow to the reservoir, and an accumulator fluidly connected to the supply conduit between the pump and the system end. The apparatus also includes a first bypass circuit including a first bypass conduit having respective ends fluidly connected to the supply conduit at a location between the pump and the supply system end and at a location between the pump and the check valve, and including a first electrically actuated valve operatively disposed in the first bypass conduit. The apparatus further includes a second bypass circuit including a second bypass conduit having respective ends fluidly connected to the supply conduit at a location between the pump and the system end and at a location between the check valve and the reservoir end, and including a second electrically actuated valve operatively disposed in the second bypass conduit.
Yet another aspect of the present invention includes a method for charging and recirculating fluid between a hydraulic system and a fluid reservoir. The method includes providing a system charging circuit including a pump with an inlet connected to the reservoir and an outlet connected to the system, and also an accumulator operatively connected to the pump outlet, the accumulator having a fluid working capacity. The pump is activated to increase fluid pressure at the pump outlet and charge fluid to the system. Fluid is selectively fed back from the circuit to the pump inlet when a pressure in the accumulator exceeds a first predetermined value. Fluid is selectively fed back from the charging circuit to the reservoir when the pressure in the accumulator exceeds a second predetermined value.